1. Field of the Invention
The present invention relates to an open cell rigid polyurethane foam and a method for producing the same, which foam prevents environmental pollution when used as heat insulators in the production of home and industrial refrigerators, and in the assembly of low temperature warehouses and the like. The invention also relates to a method for making a vacuum insulation panel using open cell rigid polyurethane foam.
2. Description of the Related Art
Recently, refrigerator manufacturers confront two problems: 1) the restriction of use of CFC which causes environmental pollution, and 2) energy saving. To solve the former problem, refrigerator manufacturers in Europe usually use cyclopentane as a blowing agent. Although cyclopentane does not destroy the ozone layer, it has a low insulation efficiency and refrigerator manufacturers in the U.S.A. more than likely will begin using a hydrofluorocarbon as a substitute blowing agent around 2003. To solve the latter problem, refrigerator manufacturers in both Japan and Europe try to save energy by applying vacuum heat insulators to refrigerators.
Generally, vacuum heat insulators are classified into a silica filler type, an open cell rigid polyurethane foam type, and a glass fiber filler type according to a fulfilled core material. Use of the silica filler type is restricted because of the dust that results in its production process and its heavy weight. Use of the glass fiber filler type is restricted because of its high production cost. The weight of open cell rigid polyurethane foam type is about 1/3 of that of the silica filler type and its mass production is simple. Consequently, many refrigerator manufacturers are trying to use the open cell rigid polyurethane foam type as a vacuum heat insulator.
A vacuum heat insulator having reduced inner pressure for the purpose of improving heat insulation properties of a refrigerator insulator, is disclosed in Japanese Patent Application Laid-open No. 64-4112, and comprises a core material enclosed by a vacuum vessel, which is made of metal-laminate film. Such a vacuum heat insulation panel uses a 100% open cell rigid polyurethane foam as a core material, covered by a metal-laminate film, for forming a high vacuum, and reduces pressure to 0.05 torr in the inner part. So, vacuum heat insulation panels whose thermal conductivity is extremely improved to 0.0065 kcal/mhr.degree.C. compared to the thermal conductivity of 0.015kcal/mhr.degree.C. of the conventional rigid polyurethane foam have been developed.
In U.S. Pat. No. 5,350,777 to Matsushita Refrigeration Company, a substitute blowing agent is used instead of chlorofluorocarbons (CFCs), in the production of conventional 100% open cell polyurethane foam. They proposed HCFCs, hydrocarbon and perchloroalkane as a blowing agent. Actually, a divalent metal salt of saturated monocarbonic acid which was used as a cell opening agent, performed its role well because of its high solubility to CFCs or HCFCs; but, hydrocarbons such as a pentane, cyclopentane and perchlorocarbon are insoluble in CFCs and HCFCs. Consequently, foaming essential for small-sized open cell rigid polyurethane foam production was not possible.